1. Field of the Invention
This invention relates generally to speech recognition systems, and more particularly, to the use of grammars for improved speech recognition.
2. Description of the Related Art
Speech recognition is the process by which an acoustic signal received by microphone is converted to a set of text words, numbers, or symbols by a computer. These recognized words may then be used in a variety of computer software applications for purposes such as document preparation, data entry, and command and control. Improvements to speech recognition systems provide an important way to enhance user productivity.
Speech recognition systems can model and classify acoustic signals to form acoustic models, which are representations of basic linguistic units referred to as phonemes. Upon receiving and digitizing an acoustic speech signal, the speech recognition system can analyze the digitized speech signal, identify a series of acoustic models within the speech signal, and derive a list of potential word candidates corresponding to the identified series of acoustic models. Notably, the speech recognition system can determine a measurement reflecting the degree to which the potential word candidates phonetically match the digitized speech signal.
Speech recognition systems also can analyze the potential word candidates with reference to a contextual model. This analysis can determine a probability that one of the word candidates accurately reflects received speech based upon previously recognized words. The speech recognition system can factor subsequently received words into the probability determination as well. The contextual model, often referred to as a language model, can be developed through an analysis of many hours of human speech. Typically, the development of a language model can be domain specific. For example, a language model can be built reflecting language usage within a legal context, a medical context, or for a general user.
The accuracy of speech recognition systems is dependent on a number of factors. One such factor can be the context of a user spoken utterance. In some situations, for example where the user is asked to spell a word, phrase, number, or an alphanumeric string, little contextual information can be available to aid in the recognition process. In these situations, the recognition of individual letters or numbers, as opposed to words, can be particularly difficult because of the reduced contextual references available to the speech recognition system. This can be particularly acute in a spelling context, such as where a user provides the spelling of a name. In other situations, such as a user specifying a password, the characters can be part of a completely random alphanumeric string. In that case, a contextual analysis of previously recognized characters offers little, if any, insight as to subsequent user speech.
Still, situations can arise in which the speech recognition system has little contextual information from which to recognize actual words. For example, when a term of art is uttered by a user, the speech recognition system can lack a suitable contextual model to process such terms. Thus, once the term of art is encountered, similar to the aforementioned alphanumeric string situation, that term of art provides little insight for predicting subsequent user speech.
Another factor which can affect the recognition accuracy of speech recognition systems can be the quality of an audio signal. Oftentimes, telephony systems use low quality audio signal to represent speech. The use of low quality audio signal within telephony systems can exacerbate the aforementioned problems because a user is likely to provide a password, name, or other alphanumeric string on a character by character basis when interacting with an automated computer-based system over the telephone.